It has been reported by H. B. Bosmann [Biochem. Biophys. Acta. 264, 339 (1972)] that glucosidase activity, particularly, .beta.-galactosidase activity is elevated in 3T3 fibroflast cells transformed by RNA tumor virus. In this connection, various researches have been made and described in, e.g., Jap. Pat. Publication No. 53-31238; The Journal of Antibiotics 28, 1006 (1975); ibid. 32 (3), 212 (1979); and ibid. 32 (3), 217 (1979). The present inventors have also investigated intensively to find the substances having .beta.-galactosidase inhibitory activity, and as the result, they have found novel .beta.-galactosidase inhibitor GT-2558 in the culture broth resulted from a strain of certain microorganisms and accomplished this invention.